The present invention relates to devices for observing an image of a specimen by using a charged particle beam and an observation method using with them and more particularly, to a means for making a optimum optical condition for observation more easier in electron holography method than in a conventional one.
The electron holography method is one of specimen observation methods by an electron microscope. The electron holography method is an observation method utilizing the nature of electrons behaving as a wave, according to which an electron beam having transmitted through a substance (object wave) is interfered with an electron beam unaffected by the substance (reference wave) to obtain an interference fringe and phase information of the electron beam is extracted from the thus obtained interference fringe. Originally, the disclosed electron holography method was contrived as a means for correcting a spherical aberration of an electron lens constituting an electron microscope but nowadays, its application range expands even to observation of a local electromagnetic field and observation of a biological specimen such as protein as well. For example, Japanese Patent Application No. 02-117544 (Corresponding to JP-A-4-14744) is available as Patent Document disclosing a general apparatus construction and the principle of image formation for realizing the electron holography.
In spite of the fact that the electron holography is known as a useful analysis method, it is practiced almost in laboratories at present and cannot be said to be materialized in the form of a device widely used as analysis appliance for practical purposes. One major reason for this is that setting of an electron optical system is complex and has a low degree of usability. To describe more specifically, the use purpose of the device quite differs depending on device users and hence conditions of wide range having difficulties in generalization must be covered, bottlenecking automation of the electron optical condition setting and besides, most of device users are skilled persons such as researchers who are prone to customize the device from the viewpoint of hardware and software and so manual operation meets user's needs rather than relying on low-grade automation.
Conventionally, adjustment of the electron optical condition in the holography electron method is carried out in accordance with the following two procedures:
(1) Deflection angle θ of an electron beam biprism is adjusted.
(2) Focal length f of an objective lens is adjusted.
Accordingly, a typical flow of adjustment of the electron optical condition in the conventional electron holography method will be described hereunder.
Conventionally, for adjustment of the electron optical system of a holography microscope, operation or work (a series of operations of searching conditions for electron holography observation) are needed as shown in a flowchart of FIG. 1. Firstly, a specimen to be observed is inserted to an electron microscope in step 81 and thereafter, in step 82, an interference fringe spacing (Δx) and an interference area (w) which satisfy a requested spatial resolution (d) are estimated. Especially when a Fourier transform method is used as a method for reconstruction of a hologram, it is important that for the sake of obtaining a desired spatial resolution, work must proceed by taking care of the necessity for the interference fringe spacing to be about ⅓ of the resolution.
In the first step of adjustment, the deflection angle θ the electron biprism provides is adjusted. In this step, for adjustment of θ, the electrical voltage to be applied to a glass fiber (biprism voltage; Vp) and relative polarity between the glass fiber and ground potential are adjusted. Ground electrodes are disposed at the opposite ends of the fiber and specifically, the polarity relative to the ground electrode is adjusted. The deflection angle θ increases in proportion to the voltage applied to the glass fiber, having an upper limit at which the interference area reaches a maximum interference area (wmax).
In branching step 83 in FIG. 1, it is decided whether Δx can be realized by adjusting Vp only. If “Yes” is determined in this step, a set Vp is applied as it is (step 84) and the program proceeds to branching step 85 in which it is decided whether an area w with a fringe spacing Δx is so wide as to accord with a requested width. If “No” in the branching step 83 or 85, exciting currents to the objective lens 5 and an intermediate lens 8 (see FIG. 2) are adjusted and an image plane or an object plane of the objective lens is moved to satisfy a requested observation condition (step 86). Through execution of a search operation in the steps 83 through 86, the image plane and focal length of the objective lens can be changed while keeping the biprism voltage Vp to the set value (step 87) and as a result, the electron optical condition can be adjusted so as to be placed in condition satisfying the desired Δx and w.
On the other hand, JP-A-2005-197165 discloses an invention according to which two electron biprisms are used to make Δx and w adjustable independently.
In order to realize a wide condition range through the method as shown in FIG. 1, the intermediate lens current needs to be adjusted highly accurately within a very narrow range. In the case of a general electron microscope, lens currents of the intermediate lens and other lenses are often preset by makers and changing to arbitrary values in the course of ordinary observation is unexpected. Therefore, in many cases, adjustment of the amounts of intermediate lens current is difficult to execute on the user side and even if an adjustment of the current amount has been done, a fine change amount adjustment is often impossible in effect. This will be detailed below.
Illustrated in FIG. 2 is a typified example of an electron optical system for electron holography observation. The electron optical system shown in FIG. 2 corresponds to that when a transmission electron microscope is used. In FIG. 2, an incident electron beam 1 irradiating a specimen 3 includes irregularity of incident angle 2 (hereinafter, this angle irregularity is termed a divergence angle). An electron beam having transmitted through the specimen passes through the objective lens 5 to form an image at the position of an objective lens imaging plane 7. During holography observation, an electron biprism 6 comprised of a ground electrode and a metal coated glass fiber of about 0.3 μm diameter is located behind the objective lens 5 and electrical potential is applied to the glass fiber. In the case of FIG. 2, by applying a negative potential, electron beam components on the right and left sides of the electron biprism 6 are deflected by an angle 11 (defection angle θ) to go away from the optical axis. As a result, the right and left electron beam components resulting from splitting the electron beam by means of the electron biprism 6 are superimposed on each other on the image plane 7 of objective lens, causing an interference fringe in an area 9. This interference fringe is called a hologram and a width of hologram area 9, when converted into a size on a specimen plane 4, has a value 10 which in turn is called an interference area w.
The interference area w can be expressed by the following equation (1) under the condition that the objective lens image plane and wavelength λ of the incident electron beam are constant. In equation (1), the second term on right side corresponds to a shading portion shielded by the electron biprism.
                    w        =                                            2              ·                              θ                ⁡                                  (                                      b                    -                    l                                    )                                                                    M                              obj                .                                              -                      2            ⁢                                          r                f                            ·                              f                                  f                  -                  l                                                                                        (        1        )            
where θ represents an angle deflected by the electron biprism, l a distance between the objective lens and the electron biprism, b a distance between the objective lens and the image plane, Mobj a magnification of the objective lens, rf a radius of the fiber of electron biprism and f a focal length of the objective lens.
Also, the value of interference fringe spacing Δx can be expressed by the following equation (2):
                              Δ          ⁢                                          ⁢          x                =                                            1                              M                                  obj                  .                                                      ·                                          λ                ⁡                                  (                                      b                    -                    f                                    )                                                                              2                  ·                  θ                                ⁢                                                                        l                    -                    f                                                                                                  =                                    f              ·              λ                                                      2                ·                θ                            ⁢                                                                l                  -                  f                                                                                                        (        2        )            
where f represents the focal length of objective lens, λ the wavelength of incident electron beam, θ the angle deflected by the electron biprism and l the distance between objective lens and electron biprism. Maximum value wmax of the area w is determined by the wavelength λ of incident electron beam and the divergence angle β. Even if superimposition by the electron biprism is effected over an area in excess of wmax to increase the interference area, the interference fringe cannot be obtained. As will be seen from equations (1) and (2), the interference fringe spacing, interference area and biprism voltage are so related to one another as to be determined uniformly if any one of them is set unless another parameter is changed. Therefore, the conventional device fails to realize enlargement or narrowness of the interference fringe spacing while keeping an interference area increased in excess of a constant limit.
When changing various parameters appearing in equations (1) and (2), the device user is requested to operate the device by using arbitrary values deviating from the condition set by the maker and is urged to operate in a very special environment. Accordingly, a highly graded technical level is required for adjustment and persons other than operators very skilled in electron microscope operation are forced to confront extremely sophisticated work.